1. Field of the Invention
The invention relates generally to wireless communications technology. More particularly, an embodiment of the invention determines signal characteristics for transmissions using Orthogonal Frequency Division Multiplexing (OFDM).
2. Background Art
Modern wireless communication systems frequently rely on the advantages of OFDM and Orthogonal Frequency Division Multiple Access (OFDMA), referred to collectively herein as OFDM(A). In particular, wireless communication systems commonly implement Institute of Electrical and Electronics Engineers (IEEE) standards such as the IEEE 802.16 standards for Wireless Metropolitan Area Networks (WMANs), including the IEEE 802.16e Draft Amendment to IEEE Standard for Local and Metropolitan Area Networks—Part 16: Air Interface for Fixed Broadband Wireless Access Systems—Physical and Medium Access Control Layers for Combined Fixed and Mobile Operation in Licensed Bands, 2004.
For the downlink (DL) mode of operation, a main reason for performance degradation in wireless systems is interference from neighboring OFDM(A) transmitters. For example, the performance degradation caused by the internal noise of a receiver is usually less than that caused by the power of an interfering signal from a nearby base station (BS). A BS or mobile units receiving OFDM(A) communications of the IEEE 802.16e type must account for interfering signals from surrounding OFDM transmitters. Previously, cancellation of these interfering signals depended on complex calculation of interference correlation matrices. The usefulness of these correlation matrices in selecting adaptive communications techniques, however, is limited to particular channel environments.
Recognizing a strongest interfering OFDM(A) signal and determining its encoding characteristics is useful, for example, in deriving information for selective interference cancellation in signal transmission. IEEE 802.16e communications send information in data frames which start with a downlink (DL) preamble. The DL preamble is transmitted in one of three defined subcarrier sets, each subcarrier set utilizing every third subcarrier starting from some predetermined number. DL preambles of an interfering signal provide a way for evaluating that signal and adjusting wireless communications to account for its effects.